1. Field of the Invention
The invention relates to a hydrostatic axial thrust bearing which includes an anti-friction bearing for supporting at least part of the bearing load when the load is too large for the pressurized fluid in the bearing.
2. Description of the Prior Art
A hydrostatic axial thrust bearing consists of a support member and a load member disposed adjacent the support member with a cavity formed between the two members to which cavity pressurized fluid is supplied for supporting the load member on the support member. The pressurized fluid is generally supplied by a pump and the pump pressures normally required are about 300 to 500 psi. Such pressures do not pose any problems, not with regard to cost nor with regard to operating noise. Occasionally, however, the loads become excessive such that fluid pressures of up to 1500 psi would be required. Pumps and bearing parts adapted to carry such a load would present substantial problems and require expensive structures hardly justifiable as such excessive loads occur generally only for short periods.
Backup anti-friction bearings have been used in connection with hydrostatic axial thrust bearings such as disclosed in U.S. Pat. No. 3,910,650. These bearings are designed to take up the bearing load when the fluid pressure is not yet sufficiently high for supporting the load, particularly during start-up. During operation of such bearings, it has been discovered that high pressure hydrostatic bearings are not immune to wear especially if there is some particulate contamination in the hydrostatic bearing oil. Although such wear occurs only very slowly, in time it affects the sensitive position of the backup anti-friction bearing relative to the limited leakage sealing surfaces such that the anti-friction bearing will take up an increasing share of the bearing load which will finally result in the dumping of the pressurized fluid from the bearing load cavity.
After many years of experience with such transmissions particularly in connection with infinitely variable traction roller transmissions especially for use in vehicles where the loads and speeds to which the bearings are subjected fluctuate within a wide range, the present inventor is able to specify requirements for a hybrid bearing, that is a hydrostatic axial thrust bearing including a backup anti-friction bearing as follows: During startup or loss of fluid pressure the anti-friction bearing must be capable of carrying the load. Normally, that is with pressurized fluid available, the hydrostatic bearing should carry most or all of the load until a maximum pressure is reached whose value is determined by design or by adjustment. Note that the hydrostatic bearing pressure is a function of the bearing load. Above a desirable limit, that is when maximum pressure is reached, the anti-friction bearing should take up any additional load. Wear on the hydrostatic bearing and seal surfaces should not affect the distribution of the load, that is wear should not affect the operation of the bearing.